Process and system for removing sulfur dioxide from flue gas

ABSTRACT

Processes and systems for producing potassium sulfate as a byproduct of a desulfurization process. Sulfur dioxide is absorbed from a flue gas using an ammonia-containing solution to produce an ammonium sulfate solution that contains dissolved ammonium sulfate. At least a first portion of the ammonium sulfate solution is heated before dissolving potassium chloride therein to form a slurry that contains potassium sulfate crystals and an ammonium chloride solution. The slurry is then cooled to precipitate additional potassium sulfate crystals, after which the potassium sulfate crystals are removed to yield a residual ammonium chloride solution that contains dissolved ammonium chloride and residual dissolved potassium sulfate. Ammonia is then absorbed into the residual ammonium chloride solution to further precipitate potassium sulfate crystals, which are removed to yield a residual ammonium chloride solution that is substantially free of dissolved potassium sulfate.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of co-pending U.S. patentapplication Ser. No. 14/923,730, filed Oct. 27, 2015. The contents ofthis prior application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to processes, systems, andequipment capable of removing gases and particulate matter and gasesfrom flue gases. The invention particularly relates to wet flue gasdesulfurization (FGD) processes, systems, and equipment with whichpotassium sulfate can be produced as a byproduct of sulfur dioxideremoval from flue gases using an ammonia-containing solution.

Gas-liquid contactors and absorbers are widely used to remove substancessuch as gases and particulate matter from combustion or flue gasesproduced by utility and industrial plants. Often of particular concernare sulfur dioxide (SO₂) and other acidic gases produced by thecombustion of fossil fuels and various industrial operations. Such gasesare known to be hazardous to the environment, and their emission intothe atmosphere is regulated by clean air statutes. Methods by whichthese gases are removed with gas-liquid contactors and absorbers havebeen referred to as wet flue gas desulfurization.

The cleansing action produced by a gas-liquid contactor is generallyderived from the passage of gas through a tower cocurrently orcountercurrently to a descending liquid that cleans the gas. Wet fluegas desulfurization processes have typically involved the use ofcalcium-based slurries or sodium-based or ammonia-based solutions.Examples of calcium-based slurries are limestone (calcium carbonate;CaCO₃) slurries and hydrated lime (calcium hydroxide; Ca(OH)₂) slurriesformed by action of water on lime (calcium oxide; CaO). Such alkalineslurries react with the acidic gases to form precipitates that can becollected for disposal or recycling. Intimate contact between thealkaline slurry and acidic gases that are present in the flue gases,such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride(HF), result in the absorption of the gases by the slurry and theformation of salts such as, in the case of calcium-based slurries,calcium sulfite (CaSO₃.½H₂O), gypsum (CaSO₄.2H₂O), calcium chloride(CaCl₂), and calcium fluoride (CaF₂). Forced oxidation of the slurry byaeration is often employed to ensure that all of the sulfites will bereacted to form sulfates, which in the case of a calcium-based slurryserves to maximize the production of gypsum.

While gas-liquid contactors and absorbers utilizing calcium-basedslurries as described above generally perform satisfactorily, theiroperation results in the production of large quantities of wastes orgypsum, the latter often having only nominal commercial value. Incontrast, ammonia-based scrubbing processes produce a more valuableammonium sulfate fertilizer. In these processes, sulfur dioxide withinthe flue gas reacts with ammonia (NH₃) to form an ammonium sulfatesolution or ammonium sulfate crystals ((NH₄)₂SO₄). A particular exampleof such a process is disclosed in U.S. Pat. No. 5,362,458 and results inthe production of ammonium sulfate fertilizer by reacting sulfur dioxideand free ammonia (NH₃) in an ammonia-containing scrubbing solution. Incertain markets, the added value of ammonium sulfate over the value ofammonia is minimal. In addition, some prior art processes have requiredbulk supplies of ammonia that are consumed by the desulfurizationprocess, necessitating the transportation and on-site storage of largequantities of ammonia. Because transportation and storage of ammonia arehighly regulated and relatively costly, under certain circumstances theproduction of ammonium sulfate using flue gas desulfurization systemshas been viewed by some in the industry as better suited for use inniche markets.

U.S. Pat. No. 5,624,649 discloses a process capable of enhancingeconomic aspects of desulfurization processes by producing a byproducthaving of greater market value than ammonium sulfate. In particular,U.S. Pat. No. 5,624,649 discloses reacting flue gases with ammonia toform an ammonium sulfate solution, and then reacting the ammoniumsulfate solution with potassium chloride (KCl) to produce potassiumsulfate (K₂SO₄) in a manner than is capable of achieving a high yield ofboth potassium and sulfate. While the process is very effective for itsintended purpose, the resulting potassium sulfate crystals may be small(for example, an average major dimension of 0.2 mm or less) andtherefore somewhat difficult to filter and subsequently handle. Inaddition, certain steps of the process involve handling a solution,slurry, or other material that may contain a high concentration of freeammonia (NH₃), which can lead to higher operating costs in order tocontain the ammonia and/or may, under some circumstances, result inammonia losses. Also, the potassium chloride salt is dissolved atambient temperature to maintain the free ammonia in solution, resultingin a relatively slow dissolution rate that may be offset in part withthe use of a relatively large and expensive reaction vessel.

From the above, it would be desirable if further advances in flue gasdesulfurization processes were available to produce potassium sulfate asa valuable byproduct.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides processes, systems, and equipment capableof producing potassium sulfate as a byproduct of a desulfurizationprocess, for example, during the removal of sulfur dioxide from fluegases produced by utility and industrial facilities.

According to one aspect of the invention, a process for removing sulfurdioxide from a flue gas includes absorbing the sulfur dioxide from theflue gas using an ammonia-containing solution to produce an ammoniumsulfate solution that contains dissolved ammonium sulfate. At least afirst portion of the ammonium sulfate solution is heated and deliveredto a vessel in which the ammonium sulfate solution dissolves potassiumchloride and first potassium sulfate crystals precipitate to form aslurry that contains the first potassium sulfate crystals and anammonium chloride solution. The ammonium chloride solution containsdissolved ammonium chloride and a first residual amount of the dissolvedpotassium sulfate. The slurry is then cooled to precipitate secondpotassium sulfate crystals from the first residual amount of thedissolved potassium sulfate in the ammonium chloride solution, afterwhich the first and second potassium sulfate crystals are removed fromthe ammonium chloride solution to yield a first residual ammoniumchloride solution that contains the dissolved ammonium chloride and asecond residual amount of the dissolved potassium sulfate. Ammonia isthen absorbed into the first residual ammonium chloride solution toprecipitate third potassium sulfate crystals from the second residualamount of the dissolved potassium sulfate in the first residual ammoniumchloride solution, and the third potassium sulfate crystals are removedfrom the first residual ammonium chloride solution to yield a secondresidual ammonium chloride solution that contains free ammonia and thedissolved ammonium chloride and is substantially free of the dissolvedpotassium sulfate.

Another aspect of the invention is a system configured and adapted withmeans for performing the steps of the process described above.

Technical effects of a process and system as described above preferablyinclude the ability to reduce the complexity and/or cost of producingpotassium sulfate as a byproduct of a desulfurization process. Inparticular, the process and system are capable of producing relativelylarge potassium sulfate crystals that can be more easily filtered andhandled, and such crystals tend to contain little if any free ammonia.As another preferred aspect, the process and system are further capableof dissolving potassium chloride at a sufficiently high temperature thatcan dramatically increase the rate of dissolution, which offers thepotential advantages of using a dissolution vessel of smaller size whilestill achieving complete dissolution of potassium chloride and theproduction of a chloride-free potassium sulfate byproduct. Furthertechnical effects include the ability to reduce risks of ammonia lossesby limiting the presence and amounts of free ammonia within the systemand process steps.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic representations of flue gas desulfurizationsystems and processes in accordance with nonlimiting embodiments of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 schematically represent flue gas desulfurization (FGD)systems and processes adapted to remove gaseous sulfur dioxide that isentrained in a flue gas through the use of an ammonia-containingsolution to produce potassium sulfate as a useful byproduct. Feedchemicals utilized in the process include sulfur dioxide (present in aflue gas), potassium chloride (potash), and ammonia. In preferredembodiments, the potassium sulfate byproduct is of high purity, fullysoluble, and in the form of large crystals that are easy to filter,handle, and use as a fertilizer. As will be discussed in reference toFIG. 1, the process and system can also produce a high concentrationammonium chloride (NH₄Cl) solution that can be used to produce ammoniumchloride crystals that are also suitable for use as fertilizer.Alternatively, as will be discussed below in reference to FIG. 2, theprocess and system can be adapted to use lime or hydrated lime torecover free ammonia from the ammonium chloride solution and produce ahighly concentrated calcium chloride (CaCl₂), which can be furtherprocessed to produce solid calcium chloride salt for variousapplications.

While the invention will be described in reference to the depicteddesulfurization systems and processes utilizing absorbers, those skilledin the art will recognize that the teachings of this invention can bereadily applied to various other desulfurization systems, includinggas-liquid contactors, scrubbing structures, and various other equipmentcapable of enabling the processes described for this invention.Furthermore, desulfurization systems and processes of this invention arecompatible with various systems capable of removing other undesirablegases, mist, dust, fumes, smoke and/or particulate matter from streamsof gases. All such alternatives and variations are within the scope ofthe invention.

FIG. 1 is a schematic view of a flue gas desulfurization system 10 inaccordance with a first nonlimiting embodiment of this invention. Asshown in FIG. 1, an absorber 12 is supplied with a flue gas through aninlet 14, and sulfur dioxide in the flue gas is reacted within theabsorber 12 to produce ammonium sulfate. More particularly, sulfurdioxide is reacted with ammonia introduced into the absorber 12 throughan inlet stream 16, with the initial reaction producing ammonium sulfite((NH₄)₂SO₃) and/or ammonium bisulfite (NH₄HSO₃). FIG. 1 depictsoxidation air as also being supplied through the inlet stream 16, whichpromotes the conversion of sulfites to sulfates through in situ forcedoxidation such that the byproduct of the reaction within the absorber 12is predominantly or entirely ammonium sulfate ((NH₄)₂SO₄) in solution.As a nonlimiting example, the absorber 12 may be operated to generate anaqueous ammonium sulfate solution that contains dissolved ammoniumsulfate. As a nonlimiting example, the aqueous ammonium sulfate solutionmay contain about 25 to about 45 weight percent dissolved ammoniumsulfate in water, with the water being present as a result of beingintroduced into the absorber 12 to provide a wash solution.

The source of the flue gas may be any process involving the combustionof fossil fuels or various industrial operations by which undesirablegases or particulate matter are produced, encompassing a wide variety ofpotential environmental pollutants and contaminants. The ammoniaintroduced into the absorber 12 through the inlet stream 16 isrepresented in FIG. 1 as being supplied with ammonia from multiplesources, which in the particular nonlimiting embodiment includes ammoniastreams 80 and 82 that draw ammonia from a pair of ammonia strippers 68and 78, the former of which may also supplemented through a makeupammonia stream 86. While a single inlet stream 16 is shown, it isforeseeable that ammonia and air could be supplied to the absorber 12separately or in other combinations through any number of streams.

The absorber 12 may physically operate in a generally conventionalmanner for the purpose of contacting the flue gas and removing sulfurdioxide therefrom, such that the sulfur dioxide is reacted to formammonium sulfate. The absorption process may involve spraying theammonia into the absorber 12 so as to provide intimate contact with theflue gas that promotes the absorption of sulfur dioxide and other acidgases, such as hydrogen chloride (HCl) and hydrogen fluoride (HF) ifpresent in the flue gas. If hydrogen chloride and/or hydrogen fluorideare present in the flue gas, as is often the case with flue gas producedby the combustion of coal, these acidic gases may be reacted within theabsorber 12 to form ammonium chloride and ammonium fluoride. Afterwardsthe scrubbed flue gas can be delivered to a stack or other suitableequipment (not shown) through an outlet 18 located at the upper end ofthe absorber 12. The absorber 12 preferably operates at high efficiencyto produce a clean flue gas that has a low sulfur dioxide content, as anonlimiting example, about 1 to about 10 percent of the sulfur dioxideintroduced in the inlet stream 14.

As taught in U.S. Pat. No. 5,624,649, the flue gases may also becontacted with an aqueous ammonium sulfate solution introduced into theabsorber 12, in which case the ammonium sulfate solution may serve as aliquid vehicle for delivering the ammonia to the absorber 12, yieldingan ammonia-containing solution. Such an ammonium sulfate solution mayalso serve to control the pH in the absorber 12 within a suitable range,for example, about 4 to 6 pH range, such that the solution is highlyreactive for high efficient capture of sulfur dioxide.

A fraction 25 of the ammonium sulfate solution produced in the absorber12 is sent to a dissolution vessel 20, to which potassium chloride (KCl)22 is also introduced and dissolved. The vessel 20 is preferably anagitated vessel or a series of vessels. The ammonium sulfate solution,shown as drawn as a stream 24 from the absorber 12, is typically at anelevated temperature, a nonlimiting example being a range of about 50 toabout 60° C. FIG. 1 represents the temperature of the ammonium sulfatesolution stream 24 as being increased by heating in a heat exchanger 26.Increasing the temperature of the ammonium sulfate solution, as anonlimiting example, to about 60 to about 120° C., serves to increasethe rate of potassium chloride dissolution in the vessel 20, therebyreducing the size of the vessel 20 required to dissolve the potassiumchloride 22.

Due to the common ion effect and low solubility of potassium sulfaterelative to that of potassium chloride, the potassium chloride 22introduced into the vessel 20 dissolves and potassium sulfateprecipitates within the vessel 20, resulting in the formation of aslurry 28 that is drawn from the vessel 20. In addition to the potassiumsulfate precipitates, the slurry 28 comprises an ammonium chloridesolution that contains dissolved ammonium (NH₄ ⁺), chloride (Cl⁻),potassium (K⁺), and sulfate (SO₄ ⁼) ions, but little if any free ammonia(NH₃). The amount of potassium chloride 22 introduced into the vessel 20is preferably controlled in such a way that the K⁺/SO₄ ⁼ mole ratio inthe vessel 20 allows for a small excess of potassium ions abovestoichiometry (2) for potassium sulfate, for example, about 2.01, insuch a way that the sulfate content of the ammonium chloride solution inthe slurry 28 is very low, preferably below 1 weight percent. Thechloride content of the potassium sulfate precipitates in the slurry 28can be minimized by ensuring complete dissolution of the potassiumchloride. The potassium sulfate precipitates that form in the slurry 28tend to be relatively large crystals, for example, preferably having anaverage major dimension of at least 0.7 mm, typically in the range ofabout 1 to 3 mm, and therefore can be readily separated from theammonium chloride solution by filtering. Significantly, the potassiumsulfate precipitates that form in the slurry 28 tend to be much largerthan those produced by the process of U.S. Pat. No. 5,624,649 as aresult of the presence of free ammonia in the solution of the latter andlittle if any free ammonia in the ammonium chloride solution that formsin the vessel 20. In the absence of free ammonia, potassium sulfatecrystals tend to be much larger due to slower precipitation.

Though the solubility of potassium sulfate is relatively low compared tothat of other salts in the slurry 28, it is estimated that roughly 10 to25 percent of the total potassium sulfate produced in the dissolutionvessel 20 remains in the ammonium chloride solution of the slurry 28.Further precipitation of potassium sulfate can be achieved by coolingthe slurry 28 with a heat exchanger 32, yielding a cooler slurry 30, asa nonlimiting example, at about 60° C. or lower, such as about 20 toabout 60° C. Because a saturated solution may result in scale onsurfaces of the heat exchanger 32, in preferred embodiments the heatexchanger 32 is a direct contact cooler with cooling air. FIG. 1 showsthe cooled slurry 30 being delivered to a filter unit 34, where thelarge potassium sulfate crystals are separated from the ammoniumchloride solution within the slurry 30 to yield a stream of potassiumsulfate crystals 36 and a filtrate (mother liquor) stream 38, the latterof which is largely an ammonium chloride solution that containsdissolved ammonium chloride (i.e., chloride and ammonium ions) but alsocontains dissolved potassium sulfate that did not precipitate in thevessel 20 and heat exchanger 32. A stream 39 of wash water can be usedto wash the potassium sulfate crystals 36 and filtrate stream 38, andthereafter the potassium sulfate crystals 36 may be sent to a dryer andto storage (not shown).

Recovery of the potassium sulfate that remains dissolved in the filtratestream 38 can be achieved by salting it out of the ammonium chloridesolution in the filtrate stream 38 using free ammonia. In the embodimentof FIG. 1, the free ammonia is introduced with an ammonia absorber 40 toyield a free ammonia-containing slurry 54, as discussed in more detailbelow. The free ammonia causes precipitation of the potassium sulfate,yielding additional potassium sulfate crystals that can be removed witha second filter unit 44 to yield a second stream (filter cake) ofpotassium sulfate crystals 42 and a free ammonia-containing filtrate(mother liquor) stream 46, the latter of which is largely an ammoniumchloride solution that contains chloride and ammonium ions but little ifany potassium sulfate (for example, less than 1 weight percent). Thepotassium sulfate crystals 42 tend to be significantly smaller than thecrystals 36 previously filtered with the first filter unit 34, forexample, an average major dimension of 0.2 mm or less, as a result ofthe presence of free ammonia in the slurry 54.

A small portion 48 of the filtrate stream 38 can be used to wash thepotassium sulfate crystals 42. The balance of the filtrate stream 38 isfed to the ammonia absorber 40, where anhydrous ammonia, introduced viaa stream 50, is absorbed in the filtrate stream 38 to provide the freeammonia for precipitating the potassium sulfate, yielding the slurry 54that contains free ammonia, as a nonlimiting example, about 5 to about30% by weight of free ammonia. FIG. 1 represents the ammonia andfiltrate stream 38 within the absorber 40 as combined with a recycledslurry 52 that contains the slurry 54 drawn from the absorber 40 and thefiltrate stream 46 exiting the second filter unit 44. Consequently, therecycled slurry 52 contains free ammonia as a result of the slurry 54and filtrate stream 46 containing free ammonia. The free ammoniaconcentration in the resulting solution within the absorber 40dramatically reduces the solubility of dissolved salts (includingpotassium sulfate) introduced into the absorber 40 by the filtratestream 38 and recycled slurry 52, such that most of the potassiumsulfate in the solution within the absorber 40 precipitates.

A portion 56 of the recycled slurry 52 is fed to the filter unit 44where the precipitated potassium sulfate crystals 42 are filtered out.FIG. 1 represents the optional use of a hydroclone 58 to concentrate therecycled slurry portion 56 and yield a concentrated slurry stream 60,which reduces the volume of the slurry fed to the filter unit 44. Theportion 48 of the filtrate stream 38 can be used to wash the resultingfree ammonia-containing filtrate stream 46 from the potassium sulfatecrystals 42, which are fed to a dissolution vessel 62 (discussed below).Except for a bleed stream 66, the filtrate stream 46 is returned to theabsorber 40 with the recycled slurry 54 as described above. FIG. 1 alsoshows an overflow stream 64 of the hydroclone 58 as being returned tothe absorber 40. As discussed below, the bleed stream 66 of the filtratestream 46 is sent to the ammonia stripper 68 for recovery of freeammonia.

Another fraction of the ammonium sulfate produced in the SO₂ absorber 12can be sent to the dissolution vessel 62 to dissolve the washedpotassium sulfate potassium sulfate crystals 42 exiting the filter unit44. The potassium sulfate crystals 42 precipitated in the absorber 40,filtered in the filter unit 44, and then dissolved in the vessel 62represents the aforementioned roughly 10 to 25% of the total potassiumsulfate that was produced in the dissolution vessel 20 but remaineddissolved in the ammonium chloride solution of the slurry 28. Inaddition to being typically smaller than the crystals 36 precipitated inthe vessel 20 and filtered in the filter unit 34, after being washed inthe filter unit 44 the crystals 42 typically have a residualconcentration of chlorides and a very low concentration of free ammonia.The potassium sulfate crystals 42 are dissolved in the dissolutionvessel 62 in a portion 70 of the stream 24 of ammonium sulfate solutiondrawn from the absorber 12. As previously noted, the ammonium sulfatesolution drawn from the absorber 12 will typically be in a temperaturerange of about 50 to about 60° C., and the temperature of the ammoniumsulfate solution may be increased to a temperature of about 60 to about120° C. by heating the solution in the heat exchanger 26. As such, thetemperature of the ammonium sulfate solution delivered to thedissolution vessel 62 is also elevated, which increases the rate ofpotassium sulfate dissolution in the vessel 62. After the dissolution ofthe potassium sulfate crystals 42, the resulting ammonium sulfatesolution can be drawn from the vessel 62 and returned to the dissolutionvessel 20 via the stream 88.

The ammonia stripper 68 that receives the bleed stream 66 of thefiltrate stream 46 for recovery of free ammonia can be a conventionalammonia stripper, for example, by utilizing steam that is directlyinjected at a lower end of the stripper 68 as live steam as shown inFIG. 1, and/or with the use of a heat exchanger reboiler (not shown).The free ammonia stripped from the bleed stream 66 preferably enters acondenser separator 72, from which the resulting anhydrous ammonia canbe recycled to the ammonia absorber 40 via the stream 50 as well asrecycled to the absorber 12 via the stream 80. As represented in FIG. 1,aqueous ammonia from the condenser separator 72 can be used as a refluxfor the ammonia stripper 68. Makeup ammonia is represented as beingadded to the absorber 40 via the makeup ammonia stream 86. Whilerepresented as being introduced through the reflux stream 74, such thatmakeup ammonia is also provided to the absorber 40 via the stream 50 andto the absorber 12 via the stream 80, the makeup ammonia can be added atvarious different locations, including directly into the absorber 40 ordirectly to the stream 50 that recycles the anhydrous ammonia to theammonia absorber 40. The amount of makeup ammonia required by the system10 should ordinarily be minimal due to very low levels of ammonia lossesfrom the system 10. As such, the desulfurization process is capable ofreducing the need to transport and store large quantities of ammonia onsite.

As noted above, the filtrate stream 46 is largely an ammonium chloridesolution that contains free ammonia, chloride, and ammonium ions andlittle if any potassium sulfate. As such, the stripper 68 produces astream 76 of an ammonium chloride solution with reduced free ammonia andvery low concentrations of potassium and sulfate ions. Further removalof residual ammonia from this ammonium chloride solution can optionallybe performed with the ammonia stripper 78 using oxidation air, forexample, supplied by a compressor 84 that also supplies the air for theinlet stream 16 to the absorber 12 as shown. The stream 82 of theresidual ammonia stripped from the stream 66 of ammonium chloridesolution can then be fed to the absorber 12 along with the oxidation airvia the inlet stream 16. In combination, the strippers 68 and 78preferably operate so that essentially all of the free ammonia (e.g.,50, 80 and 82) within the system 10 is used in the absorber 12 to absorbsulfur dioxide from the flue gas 14 and/or used in the absorber 40 toprecipitate the potassium sulfate crystals 42. The resulting stream 90obtained from the stripper 78 is an ammonium chloride solution thatcontains practically no free ammonia and may be used as a fertilizersolution or crystalized for use in a solid form.

As previously noted, FIG. 2 depicts a system 110 that offers the abilityto produce calcium chloride (CaCl₂) as an alternative or in addition toammonium chloride. The system 110 provides this capability with theaddition of an agitated dissolution vessel (or vessels) 112 in whichlime (calcium oxide; CaO) and/or hydrated lime (calcium hydroxide;Ca(OH)₂) can be used to recover ammonia from the stream 66 of ammoniumchloride solution and produce highly concentrated calcium chloride whichcan be further processed to produce solid calcium chloride salt forvarious applications. FIG. 2 shows the lime or hydrated lime as beingcombined with a bleed stream 114 drawn from the stream 76 produced bythe ammonia stripper 68. Within the vessel 112, the ammonium chloridereacts with the lime or hydrated lime to produce a lime solution, whichis then introduced into the ammonia stripper 68 to react with theammonium chloride solution therein and produce calcium chloride andammonia as follows:Ca(OH)₂+2NH₄Cl⇒CaCl₂+2NH₃+2H₂OBoth free ammonia and ammonia from the ammonium chloride and limesolutions are preferably stripped in the ammonia stripper 68, such thatthe resulting streams 76, 114, and 116 are essentially highlyconcentrated calcium chloride solutions. As was discussed in referenceto FIG. 1, further removal of residual ammonia from the ammoniumchloride solution 116 can optionally be performed with the ammoniastripper 78, such that the resulting stream 118 is essentially a highlyconcentrated calcium chloride solution that contains practically no freeammonia and may be used as a fertilizer solution or crystalized for useas a solid calcium chloride product.

EXAMPLE

Table 1 below provides an estimated material balance that is based on anammonium sulfate solution (drawn from the absorber 12 via the stream 24)that contains 1000 grams (55.556 gmole) of water and 550 grams (4.167molal) of dissolved ammonium sulfate. The ammonium sulfate solutionwithin the stream 24 is heated by the heat exchanger 26 to 100° C. andintroduced into the vessel 20, where ammonium sulfate solution dissolvesmost of the potassium chloride 22 introduced into the vessel 20. Theresulting slurry 28 contains 0.82 molal of potassium sulfate, of which88 percent is dissolved in the ammonium sulfate solution. In tandem withdissolving 8.389 molal of potassium chloride into the solution, 3.534molal of potassium sulfate precipitates. Further precipitation to atotal of 4.053 molal is achieved by cooling the slurry 28 to about 40°C. with the heat exchanger 32, after which the cooled slurry 30 isfiltered with the filter unit 34 and the resulting potassium sulfatecrystals 36 are washed. The balance of the potassium sulfateprecipitation still dissolved in the filtrate stream 38 of largelyammonium chloride (in this example, about 20 percent of the totalpotassium sulfate formed in the vessel 20) precipitates in the absorber40 where 14.705 molal of free ammonia is added to the filtrate stream38, forming the slurry 54 from which the finer potassium sulfateprecipitates are separated with the filter unit 4. The stream 76 ofresidual ammonium chloride solution contains less than 1 weight percentof the potassium and sulfate originally introduced into the system 10.

The mass balance of Table 1 shows that 550 grams of ammonium sulfateconsumed 625 gram potassium chloride to produce 705 grams of potassiumsulfate and about 1450 grams of ammonium chloride solution containing450 grams of dissolved ammonium chloride salts in 1000 grams of water.

On the basis of the above, it is believed that a commercial systemtreating 1,000,000 Nm³/hr of flue gas containing 1000 ppm of sulfurdioxide that is captured at 99% efficiency would utilize about 1500Kg/hr of ammonia to capture a total of 2828 Kg/hr of sulfur dioxide andproduce 5834 kg/hr of ammonium sulfate solution in 10,607 kg/hr ofwater. The system would also consume 6700 kg/hr of potassium chlorideand produce 7500 kg/hr potassium sulfate and about 15,000 kg/hr ofammonium chloride solution, which could be sold as-is, processed toproduce a solid, and/or treated to recover ammonia (FIG. 1) and/orprocessed to produce a calcium chloride solution or solid (FIG. 2).

TABLE 1 Free T Stream NH₄ ⁺ SO₄ ⁼ K⁺ Cl⁻ K₂SO₄ H₂O NH₃ (° C.) 24 8.3334.167 0.000 0.000 0.000 55.556 0.000 100 88 8.356 4.885 1.437 0.0230.102 55.556 0.000 100 22 0.000 0.000 8.389 8.389 0.000 0.000 0.000 3028 8.356 1.356 2.767 8.389 3.534 55.556 0.000 100 30 8.356 0.832 1.7208.389 4.053 55.556 0.000 40 36 0.000 0.000 0.000 0.000 4.053 2.000 0.00040 38 8.356 0.832 1.720 8.389 0.000 55.556 0.000 40 60 8.356 0.012 0.0808.389 0.820 55.556 14.705 40 42 0.000 0.000 0.000 0.000 0.820 0.0000.000 40 66 8.356 0.012 0.080 8.389 0.000 55.556 14.705 40 76 8.3560.012 0.080 8.389 0.000 55.556 0.000 40

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the physical configurations of the systems 10 and 110could differ from those shown, and alternative and/or additionalcomponents, materials, processes and steps other than those noted couldbe used. Therefore, the scope of the invention is to be limited only bythe following claims.

The invention claimed is:
 1. A system for removing sulfur dioxide from aflue gas, the system comprising: an absorber having an inlet forreceiving the flue gas; an inlet stream that introduces anammonia-containing solution into the absorber to absorb sulfur dioxidefrom the flue gas to produce an ammonium sulfate solution that containsdissolved ammonium sulfate; a first vessel that is connected to theabsorber, contains at least a heated first portion of the ammoniumsulfate solution received from the absorber, and is connected to asource of potassium chloride to receive the potassium chloridetherefrom, wherein within the first vessel the ammonium sulfate solutiondissolves the potassium chloride and precipitates first potassiumsulfate crystals to form a slurry that contains the first potassiumsulfate crystals and an ammonium chloride solution, the ammoniumchloride solution within the first vessel containing dissolved ammoniumchloride and a first residual amount of dissolved potassium sulfate; acooling device connected to the first vessel and through which theslurry flows for cooling the slurry to precipitate second potassiumsulfate crystals from the first residual amount of the dissolvedpotassium sulfate in the ammonium chloride solution; a first filter unitthat is connected to the cooling device, receives the ammonium chloridesolution therefrom, and removes the first and second potassium sulfatecrystals from the ammonium chloride solution to yield a first residualammonium chloride solution that contains the dissolved ammonium chlorideand a second residual amount of the dissolved potassium sulfate; anammonia absorber that is connected to the first filter unit, receivesthe first residual ammonium chloride solution therefrom, and withinwhich ammonia is absorbed into the first residual ammonium chloridesolution to precipitate third potassium sulfate crystals from the secondresidual amount of the dissolved potassium sulfate in the first residualammonium chloride solution; and a second filter unit that is connectedto the ammonia absorber, receives the first residual ammonium chloridesolution therefrom, and removes the third potassium sulfate crystalsfrom the first residual ammonium chloride solution to yield a secondresidual ammonium chloride solution that contains free ammonia and thedissolved ammonium chloride and is free of the dissolved potassiumsulfate.
 2. The system according to claim 1, further comprising a secondvessel wherein: the second vessel is connected to the second filterunit, receives the third potassium sulfate crystals from the secondfilter unit, and dissolves the third potassium sulfate crystals in asecond portion of the ammonium sulfate solution; and the second vesselis connected to the first vessel for delivering the second portion ofthe ammonium sulfate solution to the first vessel.
 3. The systemaccording to claim 1, further comprising a first stripper connected tothe second filter unit for recovering at least a portion of the freeammonia from the second residual ammonium chloride solution to produce athird residual ammonium chloride solution that contains the dissolvedammonium chloride and a reduced amount of free ammonia.
 4. The systemaccording to claim 3, further comprising a second stripper connected tothe first stripper for recovering an additional portion of the freeammonia from the third residual ammonium chloride solution to produce anammonia-free ammonium chloride solution that contains the dissolvedammonium chloride.
 5. The system according to claim 4, wherein thesecond stripper is connected to the absorber for delivering theadditional portion of the free ammonia and the air to the absorber toassist in the absorbing of the sulfur dioxide from the flue gas toproduce the ammonium sulfate solution.
 6. The system according to claim1, further comprising a third vessel connected to the second filter unitfor reacting the second residual ammonium chloride solution with limeand/or hydrated lime and a separator for recovering at least a portionof the free ammonia and ammonium ions therefrom and produce a calciumchloride solution.
 7. The system according to claim 6, furthercomprising a stripper connected to the second filter unit for recoveringany remaining portion of the free ammonia from the calcium chloridesolution to produce an ammonia-free calcium chloride solution.
 8. Thesystem according to claim 1, wherein all of the free ammonia within thesystem is either delivered to the absorber to absorb sulfur dioxide fromthe flue gas or delivered to the ammonia absorber to be absorbed intothe first residual ammonium chloride solution to precipitate the thirdpotassium sulfate crystals.